Tabular silver halide grains, their preparation and use in photographic emulsions, are widely known. Tabular silver halide grains are crystals possessing two major faces that are substantially parallel. They have been extensively studied in the literature since photographic emulsions containing these grains appeared to offer some significant advantages over photographic emulsions containing round or globular or cubic grains. Tabular grains usually have polygonal (i.e., triangular or hexagonal) parallel crystal faces, each of which is usually greater than any other crystal face of the grain and are conventionally defined by their aspect ratio (namely AR) which is the ratio of the diameter of the grain to the thickness. Tabular grains offer significant technical and commercial advantages apparent to those skilled in the art. The most important advantages of tabular grains can be summarized as follows:
1. Tabular grains have a high surface to volume ratio so that a large amount of sensitizing dye can be adsorbed on the surface, and a high development rate and covering power can be obtained. PA1 2. Tabular grains tend to lie parallel to the surface of the support base when emulsions containing them are coated and dried so that it is possible to reduce the thickness of the coated layer and accordingly to increase sharpness. PA1 3. When a sensitizing dye is added to tabular grains, the extinction coefficient of the dye is greater than the extinction coefficient for the indirect transition of the silver halide so that in X-ray materials it is possible to obtain a relevant reduction in cross-over, thereby preventing any worsening of quality. PA1 4. Tabular grains are usually very thin and so the amount of radiation absorbed per grain (proportional to the thickness) is low and there is low fogging due to natural radiation on aging. PA1 5. Tabular grains show low light scattering and the images obtained from them have a high resolution. PA1 (1) LAO-HAO-LAO PA1 (2) HAO-LAO-HAO PA1 (3) HAO-LAO-L-LAO-HAO PA1 (4) LAO-HAO-L-HAO-LAO PA1 (a) forming silver halide nuclei by adding from 5% to 15% by weight of total silver nitrate to a reaction vessel comprising a dispersing medium and bromide aqueous solution at a pBr ranging from 0 to 2 and a pH ranging from 2 to 5, PA1 (b) performing a first addition of a silver halide solvent after at least 50% by weight of silver nitrate used during nucleation has been added, PA1 (c) ripening the silver halide nuclei, PA1 (d) growing said silver halide nuclei by double jet addition of a soluble silver salt and a soluble bromide salt aqueous solutions at pBr between 1 and 2 to obtain tabular silver halide grains, PA1 (e) adjusting the pBr to a value ranging from 4.5 and 7 by single jet addition of a soluble silver salt aqueous solution, PA1 (f) performing a second addition of silver halide solvent, and PA1 (g) thickening said tabular silver halide grains by double jet addition of a soluble silver salt and a soluble bromide salt aqueous solutions at pBr between 1.0 and 3.0. PA1 (a) forming silver halide nuclei by adding from 5% to 15% by weight of total silver nitrate to a reaction vessel comprising a dispersing medium and bromide aqueous solution at a pBr ranging from 0 to 2 and a pH ranging from 2 to 5, PA1 (b) performing a first addition of a silver halide solvent after at least 50% by weight of silver nitrate used during nucleation has been added, PA1 (c) ripening the silver halide nuclei, PA1 (d) growing said silver halide nuclei by double jet addition of a soluble silver salt and a soluble bromide salt aqueous solutions at pBr between 1 and 2 to obtain tabular silver halide grains, PA1 (e) adjusting the pBr to a value ranging from 4.5 and 7 by single jet addition of a soluble silver salt aqueous solution, PA1 (f) performing a second addition of silver halide solvent, and PA1 (g) thickening said tabular silver halide grains by double jet addition of a soluble silver salt and a soluble bromide salt aqueous solutions at pBr between 1.0 and 3.0.
In spite of all these advantages, tabular grain emulsions tend toward more dispersed grain populations than can be achieved in the preparation of conventional silver halide grains. This has been a concern since reducing grain dispersion or variation in grain size within an emulsion is a basic approach to increasing the imaging consistency of the emulsion. Grain dispersion concern relates to (1) the presence of non-conforming grain shapes, such as, for example, octahedral, cubic, or rod shapes and (2) to the variance of the grain size distribution. Non-conforming grains can interact differently with light and exhibit some undesirable properties. For example, faces of non-tabular grains are randomly oriented with respect to the support base, octahedral grains exhibit lower covering power and greater thickness, and rod grains can self develop in the absence of light, thereby increasing fog.
On the other hand, even a population of grains having a common shape can have a high dispersion in terms of grain size distribution. A common method for quantifying grain size distribution is to extract a sample of individual grains, calculate the corresponding diameter for each grain (D.sub.1.fwdarw. n, wherein n is the number of extracted grains), calculate the average diameter (Dm=.SIGMA..sub.1.fwdarw. nD/n), calculate the standard deviation of the grain population diameters (S), divide the standard deviation (S) by the average diameter (Dm) and multiply by 100, thereby obtaining the coefficient of variation (COV) of the grain population as a percentage.
It is known in the art that emulsions having a low COV (e.g., lower than 30%) can be optimally sensitized as a result of their similar surface areas, have low light scattering and therefore a high image sharpness as a result of the reduction of the finer grain population, have a low granularity as a result of the reduction of the larger grain population, and have a higher contrast.
Accordingly, various solutions have been proposed in the art to reduce the COV of tabular grain emulsions. Monodispersed tabular grain emulsions and methods to prepare them are disclosed for example in U.S. Pat. No. 4,150,994, U.S. Pat. No. 4,184,877, U.S. Pat. No. 4,184,878, U.S. Pat. No. 4,301,241, U.S. Pat. No. 4,386,156, U.S. Pat. No. 4,400,463, U.S. Pat. No. 4,425,426, U.S. Pat. No. 4,797,354, U.S. Pat. No. 4,977,074, U.S. Pat. No. 4,945,037, U.S. Pat. No. 5,215,879, U.S. Pat. No. 4,798,775, U.S. Pat. No. 4,722,886, U.S. Pat. No. 4,801,522, U.S. Pat. No. 5,013,641, U.S. Pat. No. 5,254,453, EP 503,700, EP 569,075, EP 577,886, EP 588,338, EP 600,753. These patents and patent applications attempt to obtain monodispersed tabular grains by controlling various parameters during nucleation and ripening of the silver halide emulsion. The most important nucleation conditions to be kept under control for obtaining monodispersed tabular grain emulsions are temperature, gelatin concentration, addition rates of silver salt solution, addition rates of alkali halide solution, stirring rate, iodide content in the alkali halide solution, amount of silver halide solvent, pH of the dispersing medium, concentration of bromide ions in the reaction vessel, molecular weight of dispersing medium, iodide content in the vessel at the start, and the like. Similarly, the most important ripening conditions are temperature, dispersing medium concentration, silver halide solvent concentration, pBr, and addition rates of silver salt solution.
Maternaghan in U.S. Pat. No. 4,150,994, U.S. Pat. No. 4,184,877, and U.S. Pat. No. 4,184,878 describes the formation of thick monodispersed tabular grain emulsion from seed crystals having at least 90%mol of iodide.
Saito in U.S. Pat. No. 4,301,241 describes a process for forming a silver halide emulsion containing multiple twin crystal grains and a narrow grain size distribution. The examples report multiple twin crystal grain silver bromoiodide emulsions having an average grain size from 0.86 to 1.023 .mu.m and a coefficient of variation of from 11.6% to 13.6%.
Mignot in U.S. Pat. No. 4,386,156 describes silver bromide tabular grain emulsions having an aspect ratio of at least 8.5:1 and a COV of less than 30. The tabular grains described by Mignot are bounded by (100) crystal faces and are square or rectangular.
Abbot et al. in U.S. Pat. No. 4,425,426 disclose a radiographic element comprising tabular grain emulsion in which grains having thickness lower than 0.2 .mu.m, and average aspect ratio from 5:1 to 8:1, account for at least 50% of the total projected area. During precipitation of silver halide grains the rate of introduction of silver and halide salts is maintained below the threshold level at which the formation of new grain nuclei is favored in order to obtain relatively monodispersed thin tabular grains with COV lower than 30%.
Saitou et al. in U.S. Pat. No. 4,797,354 disclose a silver halide emulsion comprising hexagonal tabular grains with an "adjacent edge ratio" of from 2/1 to 1/1 accounting for 70% to 100% of the projected area of all the grains, and further that said hexagonal tabular grains are monodisperse and have an average aspect ratio from 2.5:1 to 20:1. The term "adjacent edge ratio" is referred to as the ratio of the longest edge length to the shortest edge length of each hexagonal tabular grain. Accordingly, the definition of "adjacent edge ratio" is a measure of the hexagon regularity.
Saitou et al. U.S. Pat. No. 4,977,074 disclose and claim a silver halide emulsion comprising substantially circular tabular grains with a "linear ratio" equal to or lower than 2/5 accounting for from 70% to 100% of the projected area of all the grains, and further that said circular tabular grains are monodispersed. The term "linear ratio" is defined as the ratio of the total length of the linear portion in the substantially circular tabular grain divided by the total length of the extrapolated hexagonal tabular grain. The lower the linear ratio value, the more circular the grain.
U.S. Pat. No. 4,945,037 discloses a process to produce a tabular silver halide grain emulsion in which at least 60% of the total projected area is covered by tabular grains having a core portion and an outer portion, the iodide content of the core portion being from 7 mol % to the solid solution limit. The process is characterized by specific nucleating condition, that is, a gelatin concentration of from 0.1 to 20% by weight, an addition rate of silver and halide salts of from 6*10.sup.-4 to 2.9*10.sup.-1 mol/minute per liter, and a pBr value of from 1.0 to 2.5.
U.S. Pat. No. 4,798,775 discloses a process to obtain monodispersed tabular grains comprising the steps of forming silver halide nuclei with a silver iodide content of from 0 to 5% in the mother liquor, by maintaining the pBr in the reaction vessel between 2.0 and -0.7 for at least the initial half of the nucleation time, ripening the nuclei formed in the nucleation step by maintaining the concentration of silver halide solvent from 10.sup.-4 to 5 moles per liter of mother liquor, and growing the seed grains by addition of silver and halide soluble salts or by addition of fine silver halide grains.
U.S. Pat. No. 4,801,522 discloses a process to form tabular silver halide grains having a thickness of from 0.05 to 0.5 .mu.m, average grain volume of from 0.05 to 1.0 mm.sup.3 and a mean aspect ratio higher than 2:1 comprising the steps of adding silver nitrate to a reaction vessel comprising a bromide ion concentration of from 0.08 to 0.25N (pBr=1.1-0.6), adding ammonia solution to achieve 0.002 to 0.2N after at least 2% of the total silver has been added to the vessel, and adding silver and halide (Br or Brl) salts by balanced double jet.
U.S. Pat. No. 4,722,886 describes a process to form a monodispersed tabular silver halide grain emulsion comprising the steps of adding silver nitrate to a reaction vessel comprising a bromide ion concentration of from 0.08 to 0.25N to form silver halide nuclei, adding a basic silver halide solvent (e.g., ammonia solution) to achieve 0.02 to 0.2N after at least 2% by weight of the total silver has been added to the vessel, stopping silver nitrate addition for a time period of from 0.5 to 60 minutes at a Br ion concentration of from 0.005 to 0.05N, neutralizing at least part of the present solvent, and growing the formed silver halide grains by adding silver and halide (Br or Brl) soluble salts by balanced double jet.
U.S. Pat. No. 5,013,641 describes a process of forming monodispersed silver halide emulsions comprising (a) combining silver nitrate and sodium bromide in gelatin solution, (b) adding NaOH to adjust the pH to greater than 9, (c) allowing digestion of the nucleated particles, (d) adjusting the pH to below 7 by acid addition, and (e) adding silver nitrate and sodium halide to grow the nucleated particles.
U.S. Pat. No. 5,254,453 discloses a process for forming monodispersed silver bromide or bromoiodide grains with COV lower than 25%, thickness of from 0.05 to 0.5 .mu.m, mean aspect ratio higher than 2, and diameter of from 0.2 to 3 .mu.m comprising the following steps: (a) digesting the nucleated particles in a basic silver halide solvent at a concentration of from 0.0015 to 0.015N and (b) neutralizing said basic solvent after digestion and before growing.
EP 503,700 discloses a process of forming monodispersed silver bromide or bromoiodide tabular emulsions with a COV lower than or equal to 15% characterized by the addition of an aminoazaindene at any stage of the preparation, but before 50% of the total silver halide is precipitated.
EP 569,075 discloses a process of forming monodispersed silver bromide or bromoiodide tabular emulsions with average aspect ratio higher than 2, an average thickness of from 0.15 and 0.30 .mu.m, and a COV of from 0.15 to 0.45 wherein the process is characterized by (a) providing a gelatin/bromide solution at a pBr of from 1.0 to 2.0, (b) nucleating by consuming less than 10% of the total silver nitrate used, (c) a first double jet growth (consuming at least 10% of the total silver nitrate used) at a pBr value of from 1.0 and 2.5, and (d) a second double jet growth (consuming at least 40% of the total silver nitrate used) at a pBr value higher than 2.7
EP 577,886 describes a process of forming monodispersed silver bromide or bromoiodide tabular emulsions with average-aspect ratio of from 2 to 8, and a COV lower than 30. The process comprises the following steps: (a) performing a nucleation step by balanced double jet by precipitating at most 5% of the total silver halide, (b) ripening the formed nuclei, (c) performing at least one growing step by balanced double jet at pBr lower than 2, (d) ultrafiltrating the reaction mixture during the precipitation steps with an ultrafiltration flux equal to or greater than the sum of the flow rates of the silver and halide ion solutions.
Grzeskowiak, in U.S. Pat. No. 5,028,521, discloses a process for preparing monodispersed tabular silver halide grain emulsions having an aspect ratio from 3:1 to 12:1 consisting in (a) preparing a bromide/gelatin mixture at pBr of from 0.7 to 1.0, (b) adding silver nitrate and further halide to maintain excess of bromide, (c) adding ammonia to achieve at least 0.05N after at least 20% by weight of the total silver is added, (d) adding further silver nitrate and halide by balanced double jet, by maintaining an ammonia concentration of at least 0.03N.
EP 588,338 describes a process characterized by specific nucleating condition, that comprises (a) adding from 0.30 to 9.0% by weight of the total amount of soluble silver salt to a vessel containing 0.08 to 0.25M aqueous soluble halide salt (b) adding a solution of ammoniacal base when 0.30 to 9% by weight of the total amount of soluble silver salt has been added, (c) adding soluble silver salt to obtain growth pBr of from 1.3 to 2.3, and (d) adding soluble silver and halide salts to grow tabular grains
Other recent patents and patent applications attempt to obtain monodispersed silver halide tabular emulsion by adding a specific polymeric surfactant during nucleation and/or ripening.
U.S. Pat. No. 5,215,879 describes a process to obtain monodispersed silver halide emulsions in which a polymer having the following formula is added during the ripening step. ##STR1## wherein Y is H or carboxyl group; R.sub.1 is H, a halogen atom, an alkyl group or CH.sub.2 COOM, where M is H or an alkali metal atom; L is --CONH--, --NHCO--, --COO--, OCO--, --CO--, or --O--; J is an alkylene group, an arylene group, or (CH.sub.2 CH.sub.2 O)m(CH.sub.2).sub.n, where m is an integer from 0 to 40 and n is an integer from 0 to 4; and Q is H, alkyl group, a N-containing heterocyclic group, a quaternary ammonium group, a dialkylamino group, OM, --NH.sub.2, --SO.sub.3 M, --O--PO.sub.3 M.sub.2 and --CO--R.
EP 513,722, EP 513,723, EP 513,724, and EP 513,725 describe a process in which monodispersed tabular emulsions are obtained by adding, during nucleation, polymers having the following general formulas (1) to (4), respectively.
wherein LAO is a lipophilic alkylene oxide block unit, HAO is a hydrophilic alkylene oxide block unit, and L is a trivalent or tetravalent organic group comprising nitrogen.